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FUTURE DRYER FABRICS TOWARDS HIGHER SPEEDS, CLEANING RUNNING AND IMPROVED PAPER QUALITY
As paper machine speeds increase, the requirement for total runnability control in the dryer section becomes of paramount importance. Modern production requirements face us with the ever-increasing challenge of drying paper with minimal energy use in the shortest time.
Current paper technology developments in the dryer section are therefore concentrated on accomplishing more effective energy saving drying techniques. This has necessitated improved heat transmission and ventilation effectiveness in order to avoid lengthening the dryer section. Web stability has been improved by eliminating or decreasing open draws and by utilising more advanced web stabilising equipment. In this context the dryer fabric plays an important role.
As an integral part of the papermaking process, the fabric is designed to give maximum web support and to minimise any uncontrolled air fluctuations that could cause runnability disturbances. Each fabric is specified to be compatible with machine geometry and ventilation equipment in the machine, for optimal heat and mass transfer. The dryer fabric influences the drying process and machine runnability by nature of its openness, structure, and chemical surface properties. Recent developments have made the fabric an influential tool with which we can influence, and in many cases, even control the drying process. This paper discusses the latest innovations in the field of dryer fabrics, making it possible to control the web at even higher speeds, control CD shrinkage, and minimise contamination.
Dryer Fabric Evolution
The ability to optimise the interaction between paper machine clothing and the new generation hardware in paper manufacturing plays a vital role today as machine speeds are reaching 1800 m/min. When speeds exceed 2000m/min in the near future, it will be of even greater importance.
The development of the dryer section towards high-speed production has gone through many phases during the last 20 years. A great leap was made with the installation of the first Single Run in Sweden in 1975. Development continued with the introduction of blow boxes, grooved bottom rolls, vacuum rolls, and fabric driven totally closed single tier dryer sections. The latest hardware contributions for controlled runnability at high speeds are the 2nd generation blow or suction boxes, which together with the vacuum rolls in the complete single tier configuration significantly control the sheet. (Fig 1.)
Figure 1. Total single tier dryer configuration equipped with blow boxes and vac rolls
Concurrently, the driving forces behind dryer fabric development are the constantly increasing demand for improved air handling properties, the requirements to secure web control at high speeds by minimising air drag, ventilation optimisation, and the performance of machine runnability components. There are opposing fabric concepts because, as a general rule of thumb, the more open fabrics required for optimal ventilation also carry more air, and therefore cause air flow disturbance.
During the years, a number of structural changes have taken place to dryer fabrics in order to develop more aerodynamic designs (Fig 2):
Figure 2. TDryer fabric evolution steps - The next fabric generation
All of these commercialised fabric concepts, which are defined by one or more of the properties listed above, are gaining popularity on the fastest machines in the world. The dryer sections on these machines are mainly comprised of single tier positions. Modern 100 % monofilaments fabrics today carry only about 20 % more air compared to a smooth non -textile surface at speeds of 1800 m/min. This corresponds to 1600 cubic metres hour per metre width.
Figure 3. Fabric containing 100% flat profile yarns provides improved web control
The most recent development for optimising aerodynamic properties and improving surface characteristics is the introduction of fabrics containing only flat monofilaments, both in the machine direction and in the cross machine direction. The 100 % flat monofilament structure improves the uniformity and smoothness of the fabric surface, and therefore the aerodynamic characteristics. Wind tunnel tests and pilot machine studies show that these types of AERO 2000 fabrics carry only 15 % more air than a perfectly smooth surface at a machine speed of 2000 m/min. These improved aerodynamic properties enable the use of more open fabrics to ensure more efficient operation of the runnability components.
Engineered Fabric Surfaces - A New Tool for Web
In addition to the permeability and the surface structure of the fabric, the chemical surface properties are also important factors to consider when discussing the total balance of forces acting on the running web.
For conventional dryer fabrics the impact on web contact is dominated by the contact area or land surface, and the material used in the fabric. Fabric/web contact areas for modern dryer fabrics are typically in the range of 35 - 50 %, and the governing material used is hydrophobic non-absorbent polyester. The affinity of the web to the fabric, or the sheet holding power is very similar for different standard fabrics.
The paper to fabric contact can be enhanced through the selection of fabric material. AEROGRIP is a new generation dryer fabric with an engineered surface. With AEROGRIP the chemical characteristics are shifted from its hydrophobic nature toward the hydrophilic nature of the web. If the fabric behaves more like the hydrophilic paper web, the tendency of the web to stay with the dryer fabric will increase. This improves the fabric's ability to control the web at high speeds.
The tendency for the web to lift from the fabric or slide on the fabric surface is greatly reduced. The fabric surface energy level is shifted from 40 dynes/cm to 50-60 dynes/cm for the hydrophilic surface /2/.
AEROGRIP fabrics with engineered surfaces have successfully improved runnability and web control in various applications. Typical problem areas have been sheet edge lifting, cylinder adhesion problems (Fig 4.), and sheet drop off problems on bottom rolls. AEROGRIP has also effectively improved tail control during threading, reduced sheet breaks, and eliminated sheet/fabric slippage problems in after coater sections.
Figure 4. Sheet adhesion problem on SC paper at 900 m/min eliminated by AEROGRIP
On a newsprint machine, using a 100% DIP furnish running 900 m/min, AEROGRIP eliminated the sheet drop off problem in the first single tier section. That resulted in reduced draws. The reduced draws contributed an additional 20 mm gain in sheet width. In another case AEROGRIP eliminated sheet drop off problems on vacuum rolls in the first two single tier sections and reduced sheet breaks, making it possible to increase machine speed on a 9 m wide newsprint machine running 1450 m/min.
Controlling Web CD-Shrinkage
Improving fabric to web contact by means of both the fabric surface structure and material properties is an important future contributor in controlling CD web shrinkage in single tier configurations. The combination of auxiliary machine equipment such as blow boxes and vacuum rolls, the way a machine is configured, the auxiliary equipment, and certain dryer fabrics determine the level of drying restraint.
Modifying the surface of the fabric can influence overall sheet shrinkage directly by providing an added level of restrained drying, or indirectly by generally improving runnability and thereby allowing for lower machine draws. Application of these modified fabrics to sections where maximum shrinkage occurs has resulted in added width retention. The benefits to the papermaker are both quantitative and qualitative. The qualitative effects from a more uniformly dried reel means a better product for converting operations, improved uniformity of tensile index, surface roughness, and stretch at break properties. A wider sheet offers further opportunities to the paper producers in increased production.
On a wide newsprint machine running >1500 m/min producing 45 g/m2 newsprint, AEROGRIP fabrics were installed in the 3rd and 4th single tier dryer sections in order to reduce CD shrinkage. CD shrinkage was reduced in these sections from 1.09 % to 0.67 %. The retained width in these groups was partially lost in the first open draw between the 4th single tier group and the 5th top/bottom group. However the total CD shrinkage reduction on the reel was reduced from 3.2 % to 2.7 %. (Fig 5.)
Figure 5. TCD shrinkage reduction in single tier sections using AEROGRIP fabrics
Ensuring Paper Quality in High Speed
The newest printing grade machines are today equipped with totally closed one sided single tier sections, using vacuum rolls and web stabilisers in every dryer pocket. The higher vacuum levels and the totally restrained drying process in combination with higher fabric tensions are, as discussed earlier, bringing several opportunities for more uniform paper quality. However they do increase the demand for non-marking dryer fabrics and seams.
The risk for dryer fabric marking is an important quality aspect to take into consideration both when selecting a fabric design for a certain position and when setting running conditions in the dryer section. Marking was once a consideration only for very sensitive paper grades. Today marking is becoming a common concern on many modern high-speed machines.
Typically marking can be of three different types:
Mechanical marking is seen as a plane difference in the paper surface and is a result of excessive pressure caused by the force of the fabric on the paper sheet. This can occur over the entire sheet or localised, for example in the seam area. Typical machine and fabric parameters directly influencing mechanical marking are:
Evaporative marking is not seen as a plane difference in the paper surface, but as a visual defect, especially when viewed by trans-illumination. Evaporative marking is usually a result of uneven drying due to permeability differences between the fabric and the seam area. Positions with high applied vacuums and long fabric to sheet contact times are particularly vulnerable to this type of marking. Marking caused by local, partially delayed shrinkage of the paper forms larger plane differences or waves in the paper surface. This partially delayed shrinkage is a result of uneven support, often in the seam area. Printing grades dried in total single tier dryer configurations are most sensitive for this type of marking.
The new 100 % flat monofilament technology offers very smooth fabric surfaces to eliminate any risk for fabric marking. Seam smoothness, density, and surface uniformity is becoming of great importance for new machine concepts running at high speeds. In particular surface non uniformity in the seam area has not only led to premature wear and removal, but to great concern to the paper makers and printers due to marking. This marking is in most cases due to uneven support and partially delayed shrinkage.
New very fine seam types have been developed to meet these demands and eliminate marking risks. Finer versions of the traditional pin seams with very smooth surfaces and fine loops have been developed. (A pin seam results by weaving each machine direction yarn at each end of the fabric back into the fabric and allowing that yarn to form a loop.)
The latest development is a superfine inline spiral seam. An inline spiral seam is made exactly like the pin seam except a small spiral is woven into the end of the fabric. This allows the use of twice as many MD yarns compared to the pin seam resulting in a much stronger seam. The new type of very fine in-line spiral seam, the XLNT seam, utilises a specially engineered coil shape, which exactly matches the shape of the MD monofilaments in the fabric. This together with a new seam processing technology ensures a completely smooth and dense seam area. (Fig 6.) The XLNT seam has successfully eliminated sheet marking in many recently started new high speed printing grade machines. Speciality paper machines producing sensitive decor paper have been choosing AERO2000 fabrics with only flat filaments and XLNT seams to replace needled dryer fabrics, and thereby obtained more consistent performance throughout the life of the fabric. Before these developments monofilament fabrics could not be used and only a needled fabric prevented sheet imperfections.
Figure 6. Superfine XLNT in-line spiral seam
Future Trends and Requirements
While the emphasis in dryer section development has been aimed at improved runnability for some time, the focus is now moving more and more in the direction of increased drying efficiency per dryer section length, and improved paper quality. In other words, increasing drying capacity as machine room length decreases has become a new challenge. Totally closed transfers from the press section to the dryer section, and totally closed sheet runs through the entire dryer section are required for further increases in speed. The full pocket runnability components together with vac rolls and new fabric technology insure efficient sheet support and minimise CD shrinkage.
The next major change in the dryer section for dryer fabrics will be the introduction of various high temperature drying technology concepts, such as impingement drying. Impingement drying has the potential to substantially increase drying rates for printing grades up to 150 kg/hm2, and thereby offer possibilities to reduce the machine length by as much as 25 %. (Fig 7.) High air jet temperatures up to 400o C are used in indirect impingement dryers. That exposes the fabric to continuous temperatures of 80o C running under the paper web, and up to 150 - 200oC intermittently when sheet breaks occur. These conditions require fabric and edge materials with extremely good hydrolysis and heat stability properties. Commercially available materials today are PPS, THERMONETICS, and PEEK.
Figure 7. Future high speed impingement dryer sections
Higher speeds in a totally closed silent drive single tier section will require higher dryer fabric. running tensions. Tension levels will increase to 3 - 4 kN/m, setting high demands for fabric dimensional stability and seam strength. The importance of seam surface uniformity will greatly increase at these elevated tensions if sheet marking is to be prevented.
Increased usage of recycled fibres and the requirements to maintain uniform fabric properties throughout the fabric life for optimal machine operation is bringing high interest in fabric cleaning and anti-contaminant fabric concepts. Most modern high speed machines today are equipped with cleaning for intermittent use or ultra-high pressure showers for continuous use during production. All monofilament dryer fabric structures using only flat monofilaments have proven to be favourable for staying cleaner and having easy to clean properties. Improved anti-contaminant materials containing fluoropolymers and different fabric surface modification technologies are also being developed to meet these quickly growing demands.
For many, the dryer section used to be the black box that conveyed the paper from the press section to the reel. Recent developments and forthcoming drying technology have changed many things and today the dryer section is a key part of the paper process. Many major opportunities for improvements in paper quality, production increases, and energy and cost efficiencies will be found. Future dryer fabrics will play a key role in this development.
1. Fagerholm L. "Aerodynamical Properties of Dryer Fabrics for High Speed Paper Machines", Tappi Press, 1990 Engineering Conference.
2. Luciano B. "Aerogrip, Redefining the Single Run Dryer Fabric", Albany International Fabric Facts 43, No.12.